1. Field of the Invention
The present invention relates to a double-wrap brake band apparatus for use in the automatic transmission or the like of a vehicle.
2. Related Background Art
In the automatic transmission of a vehicle or a general industrial machine, a band brake apparatus is frequently used for braking a gear changing element or a rotating element. In such a band brake apparatus, normally a frictional material is attached and bonded to an inner peripheral surface of a single steel plate which is annularly formed to serve as a brake band, and this brake band is constricted by an actuator to clamp a gear changing element or a rotating element disposed on the inner side thereof, thereby putting on a brake. Then, recently, there is proposed a double-wrap band brake apparatus employing a double-wrap brake band in order to reduce the operating force of an actuator or to enhance the braking controllability.
The double-wrap brake band 3 is, as shown in FIG. 16 and FIG. 17 (a view of a part indicated by the arrow G in FIG. 16) as being used in the automatic transmission of a vehicle, mainly comprised of an intermediate band 11 in an annular form, and a pair of annular outer bands 15 welded to a free end of the intermediate band 11 through a coupling plate 13 in a state that the respective free ends thereof oppose to each other. Onto the inner peripheral surfaces of the intermediate band 11 and the outer band 15, frictional materials 17 and 19 are respectively bonded. An anchor bracket 21 to be latched by an anchor pin 5 which is provided on the side of a main body casing 1 is welded to the operating ends of the outer bands 15 and, on the other hand, an apply bracket 25 for receiving a force in a direction in which the intermediate band 11 and the outer bands 15 are constricted by means of a push rod 23 of an actuator 7 is welded to the operating end of the intermediate band 11. In this double-wrap brake band 3, when the push rod 23 of the actuator 7 is operated in the direction indicated by the arrow in FIG. 16, the intermediate band 11 and the outer bands 15 are constricted so as to put the brake on the gear changing element (hereinafter called the drum) 27 which is fitted in the double-wrap brake band 3.
In this respect, in the conventional double-wrap brake band 3 described above, the elastic modulus of the single intermediate band 11 is naturally different from that of the paired outer bands 15. For this reason, upon reception of a load from the actuator 7, an amount of deformation of the intermediate band 11 is larger than that of the outer bands 15, so that the intermediate band 11 is moved (rotated) relatively with respect to the outer bands 15 around a joint portion therebetween. This relative movement is generated not only when a load is received at braking, but also when the initial load is applied at setting. FIGS. 18 and 19 show a state of the double-wrap brake band 3 at the setting in an exaggerated manner. As can be seen from these drawings, the intermediate band 11 is arranged such that a part thereof on the operating end is protruded inwardly with respect to the outer bands 15 by the margin m, while a part on the free end is protruded outwardly with respect to the outer bands 15 by the margin n. Also, since the rigidity of the intermediate band 11 at the joint portion with the outer bands 15 is higher than that at other parts, an amount of deformation of the operating end-side portion of the intermediate band 11 is larger than that of the free end portion. As a result, the margin m is larger than the margin n.
The double-wrap brake band 3 is produced in a variety of sizes, depending on the use or specifications thereof, and a double-wrap brake band 3 in a larger size naturally has a greater weight. However, the plate thickness of the anchor bracket 21 or the diameter of the anchor pin 5 can not be enlarged in proportion to the weight of the double-wrap brake band 3, in order to reduce the size and the weight of the brake band. For this reason, in a double-wrap brake band 3 in large size and great weight, the plate thickness of the anchor bracket 21 or the diameter of the anchor pin 5 is relatively small. Thus, the brake band 3 may be secured to the main body casing 1 unsatisfactorily, and may be inclined with respect to the drum 27.
If such a relative movement between the intermediate band 11 and the outer bands 15, or an inclination of the double-wrap brake band 3 as described above is generated, it is feared that the frictional materials 17 and 19 which are bonded to the inner peripheral surfaces of the intermediate band 11 or the outer bands 15 may come in contact with the outer peripheral surface of the drum 27. In this case, in the double-wrap brake band 3 installed in the automatic transmission of a vehicle, the frictional materials 17 and 19 are always in sliding contact with the outer peripheral surface of the drum 27 during a run of the vehicle, except when the double-wrap brake band apparatus 3 effects braking, which results in certain problems such that the fuel cost of the vehicle for running is increased and that the surfaces of the frictional materials 17 and 19 are deteriorated due to the friction heat.